Analysis of an analyte in a sample generally includes detection of the analyte. Detection may be classified as either direct or indirect. Direct detection typically includes observations that are readily apparent without any further chemical reaction. Indirect detection, on the other hand, often requires a chemical reaction before becoming readily apparent.
Indirect detection may include reactions that cause changes in mass. Devices that detect changes in mass, however, are often not amenable to certain types of samples. Another indirect detection involves events measured using labels that are radioisotopes. Such techniques, however, raise safety concerns.
Indirect detection may also include reactions that cause a change in color, fluorescence, and luminescence. Such changes, however, may be so slight that they are not readily detectable by the human eye. Slight changes in color of many known systems may be due to relatively small concentrations of analytes present in the samples. In addition, color changes and other photometric characteristics may be spread over too large an area to be very useful. Although devices and chemical reactions may be used to amplify signals, noise is often amplified as well.
Problems with detection are addressed by U.S. Pat. No. 5,518,895 issued to Thorpe et al. (May 1996). The '895 patent is directed toward a device and method that detects microorganisms in blood and other body fluids by measuring pressure changes within a sealable container. By measuring pressure changes, the accuracy of the analysis may be increased. Accuracy, however, may be negatively impacted by cleanliness and sterility of the container. Another problem may involve loss of gas caused by not sealing the container quickly enough after adding the reactant that causes the gas to be evolved.
U.S. Pat. No. 6,287,851 issued to Delwiche et al. (September 2001) teaches a sensor comprising a chamber having an inlet adapted to admit a liquid sample which gets pumped into a reaction cell using a pump. Problems exist in this system generally because it requires that a liquid sample enter the reaction cell. First, a liquid sample may not always be available. Second a sufficient volume of liquid may not be available to enable pumping to occur. Third, the existence of a pump and porous membrane to separate the liquid portion from the gaseous portion adds complexity to the sensor design.
Thus, there is a need for less complicated detection devices and methods, especially for low concentration analytes.